This invention relates to a method for addressing a matrix multiplexed thin film switched liquid crystal display (LCD) and to a LCD arrangement, particularly adapted for that method.
Copending U.S. patent application Ser. Nos. 060,290 filed July 25, 1979, now U.S. Pat. No. 4,002,308; Ser. No. 044,247 filed May 31, 1979; and Ser. No. 060,274 filed July 25, 1979, now U.S. Pat. No. 4,251,136, in the name of the present assignee, all disclose matrix multiplexed LCDs. Briefly, these LCDs consist of a pair of transparent confining plates with a layer of liquid crystal sandwiched between them. Formed on the inside surfaces of the plates are opposed row-column arrays of electrodes. To locally alter the optical transmissivity of a picture element or pel of the display, a selection voltage is applied between the appropriate pair of opposed electrodes. This has the effect of subjecting an intervening part of the LC layer to an electric field which alters a field related optical characteristic of the LC and thus changes the optical transmissivity of the LCD in that locality.
In order to reduce the number of addressing leads required, one scheme for matrix multiplexing the LCD is to interconnect the pel electrodes on one plate by electrical leads extending in the column direction and to interconnect the pel electrodes on the other plate by electrical leads extending in the row direction. In an addressing scheme for the display, a series of scan pulses V.sub.s, are, for example, applied sequentially to each of the row leads, (scan lines) while reverse polarity data pulses V.sub.d are applied periodically to selected ones of the column leads (data lines). To turn on a picture element at a selected row and column intersection, a selection voltage equal to the difference between V.sub.s and V.sub.d is made great enough to locally alter the field related optical characteristic of the LC. Non-selected pulses in a scanned line are subjected to a voltage equal to the sum of the opposite polarity voltages V.sub.s and V.sub.d.
Several factors combine to limit the number of lines that can be multiplexed in a LCD.
Firstly, at the instant a pel is selected, other, non-selected pels in the selected column also experience a pulse V.sub.d. For one address period, the rms voltage experienced by these pels is insufficient to turn them on, but if N pels in a column are switched on and off in a single field scan, a non-selected pel in that column will experience V.sub.d for N address periods. This may be enough to turn the non-selected pel on. It can be shown that the ratio of rms voltage experienced by a selected pel to that experienced by a non-selected pel is: ##EQU1## As N increases, the ratio becomes smaller and, since field effect materials such as LCs do not have a sharp threshold distinguishing on from off, the contrast ratio between selected and non-selected pels becomes poorer. At a certain number of matrix rows, the contrast ratio becomes unacceptable.
This problem is compounded for LC displays which have a narrow viewing angle. Also, since the electro-optic response of field effect materials is generally temperature dependent, then if a pel is to be off at V.sub.non-select (at high temperature), and on at V.sub.select (at low temperature), the difference between V.sub.non-select and V.sub.select must be greater than for constant temperature operation. For the above reasons, the known level of multiplexing displays is limited.
This problem can be alleviated by placing a controlling switch in series with each pel at the intersections of scan and data lines. In use, pulses V.sub.d or .vertline.V.sub.s .vertline.-.vertline.V.sub.d .vertline. do not activate a switch pel combination whereas a selection pulse .vertline.V.sub.s .vertline.+.vertline.V.sub.d .vertline. does activate the switch, whereupon the liquid crystal experiences voltage.
The copending applications mentioned previously disclose several forms of LCD controlling switches fabricated by thin film techniques, the most favoured being a switch based on a MIM (metal-insulator-metal) device which functions by tunnelling or trap depth modulation. In a typical addressing scheme for switch controlled LCD pels, a waveform for a selected pel consists of an alternating series of positive and negative pulses, a scan pulse polarity reversal in one direction coinciding with polarity reversal in the opposite direction of data pulses. Polarity reversal is necessary in order to prevent any net DC component through the LC which would result in irreversible electrochemical degradation of the LC.